Females of many mammalian species possess a more vigorous immune response to a variety of antigens than do the males. Many studies have shown that prolactin (PRL) and estradiol (E$\sb2$) augment immunity. Given that E$\sb2$ is a potent physiologic stimulus for the release of PRL from the pituitary, it is plausible that PRL mediates the influence of E$\sb2$ upon the immune system. To test this hypothesis, we conducted four experiments.In our first experiment we tested (1) if ovariectomy (OVX) resulted in a suppressed immune response and (2) if replacement of physiological levels of E$\sb2$ restored immunity in OVX rats. OVX caused a decline in antibody titers to bovine serum albumin. E$\sb2$ replacement in OVX animals did not significantly augment the antibody titers although a trend of augmentation was seen.In our second experiment we tested the effect of OVX and replacement with various tonic levels of serum E$\sb2$ upon the primary and secondary humoral response to fluorescein-keyhole limpet hemocyanin. OVX depressed antifluorescein antibody titers. Furthermore, E$\sb2$ was immunoenhancing in the physiological range and immunosuppressive in the pharmacological range of the hormone. In addition, the correlation between serum E$\sb2$ and antifluorescein antibody titers was more significant in the primary than the secondary response.In our third experiment, we investigated whether E$\sb2$ could augment antifluorescein antibody titers in OVX inbred rats in which PRL levels were suppressed by pretreatment with bromocryptine (BRC). BRC suppressed antifluorescein antibody titers in sham OVX animals. However, BRC had no detectable effect upon antibody titers in OVX + E$\sb2$ treated animals despite suppressing PRL release normally associated with E$\sb2$ injections. Furthermore, BRC unexpectedly enhanced antibody titers in OVX animals. These results suggested that modulation of the immune system by PRL may be altered by the estrogen environment.In our fourth experiment we tested the effect of daily PRL injections upon the antifluorescein response in OVX rats. Daily injections of PRL depressed the peak antifluorescein titers.We concluded that both estrogen and PRL modulate humoral immunity but that the modulation of immunity by PRL may depend upon the estrogen environment. To what extent PRL mediates estrogen's effects upon immunity is still not known.